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Document labeled break and continue in the reference manual #3870

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Document labeled break and continue in the reference manual #3870

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41 changes: 28 additions & 13 deletions doc/rust.md
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Expand Up @@ -1951,32 +1951,47 @@ while i < 10 {

### Infinite loops

A `loop` expression denotes an infinite loop:
The keyword `loop` in Rust appears in two different forms.
The first one denotes an infinite loop,
while the second one is described in [Loop expressions](#loop-expressions):

~~~~~~~~{.ebnf .gram}
loop_expr : "loop" '{' block '}';
loop_expr : "loop" [ ident ':' ] '{' block '}';
~~~~~~~~

A `loop` expression may optionally have a _label_.
If a label is present, then labeled `break` and `loop` expressions nested within this loop may exit to the top level.
See [Break expressions](#break-expressions).

### Break expressions

~~~~~~~~{.ebnf .gram}
break_expr : "break" ;
break_expr : "break" [ ident ];
~~~~~~~~

Executing a `break` expression immediately terminates the innermost loop
enclosing it. It is only permitted in the body of a loop.
A `break` expression has an optional `label`.
If the label is absent, then executing a `break` expression immediately terminates the innermost loop enclosing it.
It is only permitted in the body of a loop.
If the label is present, then `break foo` terminates the loop with label `foo`,
which need not be the innermost label enclosing the `break` expression,
but must enclose it.

### Loop expressions

~~~~~~~~{.ebnf .gram}
loop_expr : "loop" ;
~~~~~~~~

Evaluating a `loop` expression immediately terminates the current iteration of
the innermost loop enclosing it, returning control to the loop *head*. In the
case of a `while` loop, the head is the conditional expression controlling the
loop. In the case of a `for` loop, the head is the call-expression controlling
the loop.
loop_expr : "loop" [ ident ];
~~~~~~~~

A `loop` expression (the second form of the `loop` keyword) also has an optional `label`.
If the label is absent,
then executing a `loop` expression immediately terminates the current iteration of the innermost loop enclosing it,
returning control to the loop *head*.
In the case of a `while` loop,
the head is the conditional expression controlling the loop.
In the case of a `for` loop, the head is the call-expression controlling the loop.
If the label is present, then `loop foo` returns control to the head of the loop with label `foo`,
which need not be the innermost label enclosing the `break` expression,
but must enclose it.

A `loop` expression is only permitted in the body of a loop.

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